Sound is kinetic energy released by the vibration of molecules in a medium, such as air. In industrial applications, sound may be generated in any number of ways or in response to any number of events. For example, sound may be generated in response to vibrations resulting from impacts or frictional contact between two or more bodies. Sound may also be generated in response to vibrations resulting from the rotation of one or more bodies such as shafts, e.g., by motors or other prime movers. Sound may be further generated in response to vibrations caused by fluid flow over one or more bodies. In essence, any movement of molecules, or contact between molecules, that causes a vibration may result in the emission of sound at a pressure level or intensity, and at one or more frequencies.
The use of unmanned aerial vehicles such as airplanes or helicopters having one or more propellers is increasingly common. Such vehicles may include fixed-wing aircraft, or rotary wing aircraft such as quad-copters (e.g., a helicopter having four rotatable propellers), octo-copters (e.g., a helicopter having eight rotatable propellers) or other vertical take-off and landing (or VTOL) aircraft having one or more propellers. Typically, each of the propellers is powered by one or more rotating motors or other prime movers.
With their ever-expanding prevalence and use in a growing number of applications, unmanned aerial vehicles frequently operate within a vicinity of humans or other animals. When an unmanned aerial vehicle is within a hearing distance, or earshot, of a human or other animal, noises generated by the unmanned aerial vehicle during operation may be detected by the human or the other animal. Such noises may include, but are not limited to, sounds generated by rotating propellers, operating motors or vibrating frames or structures of the unmanned aerial vehicle. Depending on the sizes of an unmanned aerial vehicle's propellers, the operational characteristics of its motors or the shapes or dimensions of its frame or structure, the net effect of the noises generated by the unmanned aerial vehicle may be annoying at best, or deafening at worst.